Method to form a film for protection against corrosion on sheets of galvanized steel



United States Patent METHOD TO FORM A FILM FOR PROTECTION AGAINST (IORROSION 0N SHEETS OF GAL- VANIZED STEEL Giovanni Odone and Giovanni Milanese, Genoa, Italy, assignors to Cornigliano Societa per Azioni, Genoa, Italy No Drawing. Filed Oct. 5, 1959, Ser. No. 844,154

Claims priority, application Italy Apr. 1, 1959 6 Claims. (Cl. 148-621) It is Well known that the galvanized sheets when stored, without a suitable control of humidity and temperature, are covered after a certain time by a layer of corrosion products usually called white rust. Such rus-t is constituted by basic carbonates of zinc of variable composition. in a first time, the corrosion is an electrochemical phenomenon, which afterwards is completed by a chemical reaction. First, it takes place a formation of zinc hydrate, due to an electrochemical action of dilferential oxidation of the zinc layer and caused by a not-uniform exposure to the air of the zinc layer. In fact, when the galvanized sheets are piled in a damp warehouse, the velocity of corrosion is much higher than when they are left freely exposed to the air. The air humidity acts as an electrolyte in the corrosion processes together dissolved oxygen of the air. It is so formed an electrochemical asymmetrical system, under form of an oxygen concentration couple, with the corresponding formation of an electromotive force. The areas with the highest concentration of oxygen, that is the ones more exposed to the air, act as cathodic areas, while the areas less exposed to the air, act as anodic areas.

There will form then an unsolvable precipitate of zinc hydrate Zn(Ol-l) according to the following reaction:

In a second time the carbon dioxide changes the zinc hydrate into zinc basic carbonate of variable composition.

The present invention has as object a passivating method and chemical solution suitable to form a protection film against corrosion of the zinc layer applied on the steel both in coil and in sheet form, in order to prevent the formation of basic carbonate of zinc as above stated.

The passivating method for the protection against corrosion of galvanized steel both in coil and in sheet form according to the present invention, is as follows: after cooling and crystallization of the layer of the melted zinc, galvanized coils are uniformly wetted on both surfaces with a passivating solution formed by an oxidizing compound of an element in the hexavalent state as chromium anhydride; a mineral acid, as sulphuric acid, and a surface active agent (Wetting agent). Such a solution, chemically acting on the outer surface of the zinc, forms on it a very thin film of a non-metallic compound, insensible to the action of the atmospheric agents, which prevents or at least slows down the reaction between the zinc and the atmospheric agents.

The above passivating method is also characterized in that the passivating solution suitable to form the protection film is used hot at a temperature between 60 C. and 90 C.

The passivating solution suitable to realize the above method and to obtain a passivating protection against corrosion of the zinc layer, according to the present invention, is an aqueous solution composed as follows:

From 7 to 12 grams of an oxidizing compound of an element in the hexavalent state, as chromium anhydride technically pure, preferably 8 grams/liter.

From 0.05 to 0.20 milliliter of sulphuric acid, sp. gr. 1.84,

preferably 0.15 nil/liter.

From 0.20 to 0.35 milliliter of a surface active agent (wetting agent), preferably 0.25 mL/liter.

3,039,990 Patented June 19, 1962 In order to obtain the said passivating protection are necessary the two following preliminary conditions:

(a) The possibility of wetting the surface to be treated which may be obtained by adding a surface active agent (wetting agent) to the passivating solution.

(b) The activation of the surface, which may be obtained by adding to the passivating solution small amounts of a mineral acid as sulphuric acid, which acts as an activation agent, that is it makes easier the reaction between the surface of the metal and the passivating solution.

The mechanism of the above passivating protection against corrosion results from a whole of physico chemical phenomena very complicated which involves the surface of the metal contacting the solution. As a final result it is obtained the formation on the metal of a film of basic compounds of chromium having high hydrophobic characteristics.

The galvanized steel both in coil or in sheet form, subjected to the passivating method against corrosion according to the present invention, has the following characteristics:

(1) Remarkable preservation of the protecting layer;

(2) No change in the original brilliance of the galvanized product;

(3) Preservation of the original brilliance for up to a year in Warehouse without the adoption of special precautions to control humidity and temperature of warehouses.

The method of application of the passivating solution for the protection of galvanized steel to the present invention may be easy accomplished without the need of expensive equipments.

Although for describing reasons the present invention has been described on the ground of what above stated,

F many modifications and changes may be made in embodying the invention, as for the both temperature at which the passivating bath is used, and the composition of the solution used in the bath, all modifications and changes however being based on the main ideas of the invention as resumed in the following claims.

What we claim is:

1. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solution simultaneously containing chromium trioxide, sulfuric acid, and a surface active agent, said solution being effective to form upon the surface of the zinc a thin film resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

2. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solution at a temperature of 60 to C. and simultaneously containing chromium trioxide, sulfuric acid, and a surface active agent, said solution being effective to form upon the surface of the zinc a thin film resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

3. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly Wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solution simultaneously containing about 8 grams per liter of chromium trioxide, about 0.15 milliliter per liter of sulfuric acid, and about 0.25 milliliter per liter of a surface active agent, said solution being effective to form upon the surface of the zinc a thin film resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

4. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solu tion at a temperature of 60 to 90 C. and simultaneously containing about 8 grams per liter of chromium trioxide, about 0.15 milliliter per liter of sulfuric acid, and about 0.25 milliliter per liter of a surface active agent, said solution being etfective to form upon the surface of the zinc a thin film resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

5. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solution simultaneously containing about 7 to 12 rams per liter of chromium trioxide, about 0.05 to 0.20 milliliter per liter of sulfuric acid of a specific gravity of about 1.84, and about 0.2 to 0.35 milliliter per liter of a surface active agent, said solution being etfective to form upon the surface of the zinc a thin film. resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

6. A method of passivating galvanized steel to protect it against corrosion which comprises uniformly wetting the surfaces of said steel, after cooling and crystallization of the layer of melted zinc thereon, with an aqueous solution at a temperature of about 60 to 90 C. and simultaneously containing about 7 to 12 grams per liter of chromiurn trioxide, about 0.05 to 0.20 milliliter per liter of sulfuric acid of a specific gravity of about 1.84, and about 0.2 to 0.35 milliliter per liter of a surface active agent, said solution being effective to form upon the surface or" the zinc a thin film resistant to the action of atmospheric agents, whereby to retard reaction between said zinc and said atmospheric agents.

References Cited in the file of this patent UNITED STATES FATENTS 2,502,476 Ott et al Apr. 4, 1950 2,559,878 Johnson July 10, 1951 2,610,133 Thompson Sept. 9, 1952 FOREIGN PATENTS 586,517 Great Britain Mar. 21 1947 

1. A METHOD OF PASSIVATING GALVANIZED STEEL TO PROTECT IT AGAINST CORROSION WHICH COMPRISES UNIFORMLY WETTING THE SURFACES OF SAID STEEL, AFTER COOLING AND CRYSTALLIZATION OF THE LAYER OF MELTED ZINC THEREON, WITH AN AQUEOUS SOLUTION SIMULTANEOUSLY CONTAINING CHROMIUM TRIOXIDE, SULFURIC ACID, AND A SURFACE ACTIVE AGENT, SAID SOLUTION BEING EFFECTTIVE TO FORM UPON THE SURFACE OF THE ZINC A THIN FILM RESISTANT TO THE ACTION OF ATMOSPHERIC AGENTS, WHEREBY TO RETARD REACTION BETWEEN SAID ZINC AND SAID ATMOSPHERIC AGENTS. 